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Hibernate configuration file working principle and two designs: one-to-many and many-to-many

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2018-07-25 15:28:201411browse

"One-to-many" is the most common mapping relationship, which is simply like the relationship between consumers and orders. One-to-many: From the consumer's perspective, a consumer can have multiple orders, which is one-to-many. Many-to-one: From an order perspective, multiple orders can correspond to one consumer, which is many-to-one. A brief personal summary.

One-to-many

Entity class

一: private Set<LinkMan> linkMans; // 表达一对多关系
多: private Customer customer ; //表达多对一关系

Configuration file

One:

    <!-- 集合,一对多关系,在配置文件中配置 -->
    <!-- 
        name属性:   集合属性名
        column属性: 外键列名
        class属性:  与我关联的对象完整类名
     -->
     <!-- 
         级联操作: cascade属性
             save-update: 级联保存更新
             delete:      级联删除
             all:         save-update+delete
         级联操作: 简化操作.目的就是为了少些两行代码.
      -->
      <!-- inverse属性: 配置关系是否维护. 
                 true:          customer不维护关系
                 false(默认值): customer维护关系
          inverse属性: 性能优化.提高关系维护的性能.
          原则: 无论怎么放弃,总有一方必须要维护关系.
          一对多关系中: 一的一方放弃.也只能一的一方放弃.多的一方不能放弃.
      -->
    <set name="linkMens" inverse="true" cascade="delete">
        <key column="lkm_cust_id" ></key>
        <one-to-many class="LinkMan" />
    </set>

Many:

    <!-- 多对一 -->
    <!-- 
        name属性:   引用属性名
        column属性: 外键列名
        class属性:  与我关联的对象完整类名
     -->
     <!-- 
         级联操作: cascade属性
             save-update: 级联保存更新
             delete:      级联删除
             all:         save-update+delete
         级联操作: 简化操作.目的就是为了少些两行代码.
      -->
      <!-- 多的一方: 不能放弃维护关系的.外键字段就在多的一方.  -->
    <many-to-one name="customer" column="lkm_cust_id" class="Customer"></many-to-one>

Many-to-many

Entity class

多:private Set<User> users; // 表达多对多关系
多:private Set<Role> roles; // 表达多对多关系

Configuration file
Many:

    <!-- 多对多关系表达 -->
    <!-- 
        name: 集合属性名
        table: 配置中间表名
        key
         column: 外键,别人引用"我"的外键列名
        many-to-many
         class:   我与哪个类是多对多关系
         column:  外键.我引用别人的外键列名
     -->
    <!-- 使用inverse属性
        true: 放弃维护外键关系
        false(默认值):维护关系
        
    结论: 将来在开发中,如果遇到多对多关系.一定要选择一方放弃维护关系.
         一般谁来放弃要看业务方向. 例如录入员工时,需要为员工指定所属角色.
         那么业务方向就是由员工维护角色. 角色不需要维护与员工关系.角色放弃维护
     -->        
    <set name="users" table="sys_user_role" inverse="true" >
        <key column="role_id" ></key>
        <many-to-many class="User" column="user_id" ></many-to-many>
    </set>

Many:

    <!-- 多对多关系表达 -->
    <!-- 
        name: 集合属性名
        table: 配置中间表名
        key
         column: 外键,别人引用"我"的外键列名
        many-to-many
         class:   我与哪个类是多对多关系
         column:  外键.我引用别人的外键列名
     -->
     <!-- cascade级联操作:
                 save-update:  级联保存更新
                 delete:       级联删除
                 all:          级联保存更新+级联删除
         结论: cascade简化代码书写.该属性使不使用无所谓. 建议要用只用save-update.
              如果使用delete操作太过危险.尤其在多对多中.不建议使用.
     -->
    <set name="roles" table="sys_user_role" cascade="save-update" >
        <key column="user_id" ></key>
        <many-to-many class="Role" column="role_id" ></many-to-many>
    </set>

Related recommendations:

Several forms of many-to-many relationships in database design_MySQL

Configuration and management of MySQL multiple instances in detail

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